In the structure of `HNO_(3)`, why is N-O bond (121 pm) shorter than N-OH bond (140 pm) ?

Explanation

`HNO_(3)` is a resonance hybrid of the following two canonical structures :

Due to resonance N-O bond has some double bond character. In contrast, N-OH has only pure single bond character. Since single bonds are longer than double bonds, therefore, N-OH bond is longer (140 pm) than N-O bond (121 pm) in `HNO_(3)`.